The cobamide-dependent ribonucleotide reductase of Lactobacillus leichmannii has been purified to the point of apparent homogeneity. The purified enzyme catalyzes a transfer of hydrogen from a thiol reductant such as dihydrolipoate to the 2' deoxyribosyl carbon of dCTP, cobamide coenzyme serving as an intermediate acceptor-donor of hydrogen. The enzyme is allosterically regulated by the various deoxyribonucleoside triphosphates, substrate specificity being wholly dependent on the allosteric effector present. Current experiments seek to study (1) physical properties and subunit structure of the enzyme; (2) validity of the "methylfolate trap" theory on vatamin B12 action; (3) levels of "methyltransferase" and other enzymes catalyzing interconversions of folate compounds and hematopoietic compounds; and (4) studies of the synthesis and degradation of pteroylpolyglutamates in animal cells.